One conventional optical head comprises a collimating lens for collimating laser beam radiated from a semiconductor laser, a beam splitter for transmitting the laser beam therethrough and for reflecting beam reflected from an optical disk, a total reflection prism for reflecting the beam transmitted through the beam splitter and the optical disk-reflected beam to bend a light path thereof by 90.degree., a first focusing lens for focusing the beam on the optical disk, a 1/2 wavelength plate for rotating a polarization of the beam reflected from the beam splitter by 90.degree., a second focusing lens for focusing the beam transmitted through the 1/2 wavelength plate, a polarizing beam splitter for transmitting and reflecting the beam focused by the second focusing lens to produce two beams having polarizations orthogonal to each other, a two-divided photodetector for receiving the beam transmitted through the polarizing beam splitter, a cylindrical lens for transmitting the beam reflected by the polarizing beam splitter to produce beam of astigmatism wave surface, and a four-divided photodetector for receiving the astigmatism wave surface beam.
In the conventional optical head, a tracking error is detected in a push-pull method by using signals of the two-divided photodetector, a focusing error is detected in an astigmatism method by using signals of the four-divided photodetector, and a RF signals are obtained in accordance with a predetermined calculation based on the signals of the two and four-divided photodetectors, although the details thereof and operation will be explained in more detail later.
However, the conventional optical head has a relatively large size, for example, greater than 40.times.40.times.30 mm.sup.2, and thus this prevents miniaturization and lightening of the entire optical disk system.
Further, a push-pull method has generally been utilized for detecting a tracking error. Therefore, when the first focusing lens is moved in the direction normal to its optical axis by means of an actuator on the basis of the tracking error signals, the deviation may arise between the optical axis of the first focusing lens and the dividing line of the two-divided photodetector for detecting the tracking error to induce the imbalance between two quantities of light which are incident to two photodetector elements respectively. As a result, a direct current offset is generated in the tracking error signals, thereby resulting in a limited range of the tracking error controlling.
Furthermore, there is the interference among a RF signal, a focusing error signal and a tracking error signal to cause a servomotor to be unstable.
In addition, such a conventional optical head has a number of optical parts required to be optically polished, and thus this is expensive and their adjustments are complex and cumbersome.